Instead, avoid creating such Result nodes in the first place, by making
plan_pushdown_tlist() check if the Result node would have any work to do.

With this, you get Result nodes in some cases where the old code could zap
it away. But on the other hand, this can avoid inserting Result nodes, not
only on top of Appends, but on top of any node. This can be seen in the
included expected output changes: some test queries lose a Result, some
gain one. So performance-wise this is about a wash, but this is simpler.

The reason to do this right now is that we ran into issues with the
"zapping" code while working on the 9.0 merge. I'm sure we could fix those
issues, but let's do this rather than spend time debugging and fixing the
zapping code with the merge.